1. Structures

2. Structure Basics
A structure is a collection of data values, called data members, that form a single unit.
Unlike arrays, the data members can be of different types.

3. Structure Definition struct name { variable declaration; . };
the keyword struct announces that this is a structure definition

4. Initializing Structures
struct Rect {
double x;
double y;
char color;
double width;
double height; };
struct rect r1 = {0,0,’r’,5,10}; x y r
color 5
width 10
height

5. Alternate Declaration
typedef struct {
double x;
double y;
char color;
double width;
double height;
} rect;
rect r1 = {0,0,’r’,5,10}; x y r
color 5
width 10
height

6. Accessing Data Members
To access a data member use
Varname.membername
struct rect r1 = {0,0,’r’,5,10};
r1.x
r1.y
r1.color
r1.width
r1.height x y r
color 5
width 10
height

7. Accessing Data Members
To access a data member with a pointer use
varname->membername
struct rect r1 = {0,0,’r’,5,10};
struct rect *rptr=&r1;
rptr->x
rptr->y
rptr->.color
rptr->width
rptr->height x y r
color 5
width 10
height

8. Assignment operator
Assignment operator is defined for structure of the same type.
struct rect {
double x;
double y;
char color;
double width;
double height; };
struct rect r1, r2
r1.x = 10.5;
r1.y = 18.99;
r1.width = 24.2;
r1.height = 15.9;
r1.color = 'r';
/* Copy all data from r1 to r2. */
r2 = r1;

9. Scope of a Structure Member variables are local to the structure.
Member names are not known outside the structure.

10. Exercise (a,b)+(c,d)=(a+c,b+d) (a,b)*(c,d)=(a*c,b*d)
Write a program using structures that manipulates pairs. Addition and multiplication of pairs is defined below.
(a,b)+(c,d)=(a+c,b+d)
(a,b)*(c,d)=(a*c,b*d)

11. Pair Structure
Store two integers to represent the first and second number of pair
struct pair {
int first;
int second; };

12. Addition struct pair add(struct pair p1, struct pair p2) {
struct pair temp;
temp.first = p1.first + p2.first;
temp.second = p1.second + p2.second;
return temp; }

13. Multiplication struct pair multiply(struct pair p1, struct pair p2) {
struct pair temp;
temp.first = p1.first * p2.first;
temp.second = p1.second * p2.second;
return temp; }

14. How to use the functions
struct pair mp1,mp2,mp3,mp4;
printf("Enter first pair\n");
scanf("%d%d",&mp1.first, &mp1.second);
printf("Enter second pair\n");
scanf("%d%d",&mp2.first, &mp2.second);
mp3 = add(mp1,mp2);
printf("Addition result = (%d,%d)\n",mp3.first,mp3.second);
mp4 = multiply(mp1,mp2);
printf("Multiplication result = (%d,%d)\n",mp4.first,mp4.second);

15. Exercise Update the program to support the following on pairs
c*(a,b) = (c*a,c*b)
(a,b)^c = (a^c,b^c)

16. Structure Representation & Size
struct CharCharInt {
char c1;
char c2;
int i; };
struct CharCharInt foo;
foo.c1 = ’a’;
foo.c2 = ’b’;
foo.i = 0xDEADBEEF;
sizeof(struct …) =
sum of sizeof(field)
+ alignment padding
Processor- and compiler-specific 62 61 EF BE AD DE c1 c2 i
padding
x86 uses “little-endian” representation

17. Structure Representation & Size
struct Rect {
double x;
double y;
char color;
double width;
double height; };
struct Rect r1 = {0,0,’r’,5,10};
sizeof(r1) =
sizeof (double)
sizeof (char)
+ alignment padding
Processor- and compiler-specific
sizeof(r1) is doubles (8 bytes each)+1 char (1 byte) + 3 bytes padding

18. Using malloc with structures
struct rect *rptr;
rptr = (struct rect*)malloc(sizeof(struct rect));
rptr->x = 10.5;
rptr->y = 18.99;
rptr->width = 24.2;
rptr->height = 15.9;
rptr->color = 'r';
print(*rptr);
free(rptr);

19. Arrays of Structures ……….......
Arrays of structures may be declared in the same way as other C data types.
struct rect rectangles[20];
rectangles[0] references first structure of rectangles array.
rectangles[0].color = ‘r’; x y
………
color
width
height 1 2 3 19

20. Structures as Arguments to Functions
When a structure is passed as an argument to a function, it is a call-by-value reference.
Changes made to the formal parameters do not change the argument.
A pointer to a structure may also be passed as an argument to a function.
Changes made to the formal parameters also change the argument.

21. Call by Value Example s s1 Updated s struct simple { int ival;
double dval; };
void fun1(struct simple s)
s.ival = s.ival+1;
s.dval = s.dval + 2; }
int main(void)
struct simple s1 = {10, 1.5};
fun1(s1);
printf(“%i %lf\n”, s1.ival , s1.dval );
return 0; s s1 10 10
ival
1.5
1.5
dval
Updated s 11
3.5

22. Call by Reference Example
struct simple {
int ival;
double dval; };
void fun1(struct simple *s)
s->ival = s->ival+1;
s->dval = s->dval + 2; }
int main(void)
struct simple s1 = {10, 1.5};
struct simple *sptr = &s1;
fun1(sptr);
printf(“%i %lf\n”, sptr->ival , sptr->.dval );
return 0;
sptr s1 10
ival
1.5
dval s
Updated s1 11
3.5

23. Nested Structures
Structure definitions may contain data members that are other structures:
struct Card {
char suit;
int rank; };
struct Deck
struct Card cards[52];
int next_card = 0;
Card
suit
rank 1 51
…...
cards ……
next_card

24. Initialize the Deck for (i=0; i<13; i++) for (i=26; i<39; i++) {
d1.cards[i].suit = 'h';
d1.cards[i].rank = (i-26)+1; }
for (i=39; i<52; i++)
d1.cards[i].suit = 's';
d1.cards[i].rank = (i-39)+1;
for (i=0; i<13; i++) {
d1.cards[i].suit = 'c';
d1.cards[i].rank = i+1; }
for (i=13; i<26; i++)
d1.cards[i].suit = 'd';
d1.cards[i].rank = (i-13)+1;

25. Print the Deck void print(struct Deck d1) { int i;
for (i=0; i<52; i++)
printf("%c %d\n", d1.cards[i].suit, d1.cards[i].rank);
return; }

26. Swap Two Cards from Deck
void swap(struct Deck *ptr) {
int x,y;
struct Card temp;
x = rand() % 52;
y = rand() % 52;
temp = ptr->cards[x];
ptr->cards[x] = ptr->cards[y];
ptr->cards[y] = temp;
return; }

27. How to Shuffle the Deck? for (i=0; i<COUNT; i++) {1 2 3 51
…………………………
for (i=0; i<COUNT; i++) {
// swap two cards from Deck }

28. BlackJack Shuffle the deck Deal first 2 cars to user
Print both cards on screen
Next two cards to dealer
Print only the first on screen
Ask user whether he/she wants to continue
Highest total <= 21 wins
Jack, Queen, King are 10 points
Ace is 11 points
Other cards represented by their number

29. Unions EF BE AD DE Choices: An element is sizeof(union …) =
union AnElt {
int i;
char c; };
union AnElt elt1, elt2;
elt1.i = 4;
elt2.c = ’a’;
elt2.i = 0xDEADBEEF;
Choices:
An element is
an int i or
a char c
sizeof(union …) =
maximum of sizeof(field) EF BE AD DE c i
padding

30. ? Unions A union value doesn’t “know” which case it contains
How should your program keep track whether elt1, elt2 hold an int or a char? ?
union AnElt {
int i;
char c; };
union AnElt elt1, elt2;
elt1.i = 4;
elt2.c = ’a’;
elt2.i = 0xDEADBEEF;
if (elt1 currently has a char) …
Basic answer: Another variable holds that info

31. Static Array Allocation
struct rect rectangle[100];
Allocates space for 100 rectanges
Each rectangle requires 36 bytes
Allocates 3600 bytes total

32. Array of Pointers struct rect *rectptr[100];
Allocates an array of 100 pointers
Each pointer is 4 bytes
Allocates 400 bytes total
Pointers do not point to allocated space

33. Array of Pointers for (i=0; i<100; i++) rectptr[i] = NULL;
Initializes pointers to NULL

34. Array of Pointers for (i=0; i<100; i++)
rectptr[i] = (struct rect*)malloc(sizeof(struct rect));
Allocate space so that each pointer points to a rectangle
You can allocate space when you need it
Allocate space for 10 rectangles if you just need 10

35. Array of Pointers for(i=99; i>=0; i--) free(rectptr[i]);
Free the space allocated using malloc
If you don’t free all the space, it will cause memory leak
Space will appear as used although it is not used
Used valgrind command to find memory leaks

36. Array of Pointers void test2(struct rect *a[]) { struct rect *temp;
temp = a[0];
a[0] = a[1];
a[1] = temp; }
Swaps first 2 pointers
Changes are permanent since arrays are pass by reference

37. Array of Pointers void test(struct rect *a[]) { struct rect temp;
temp = *a[0];
*a[0] = *a[1];
*a[1] = temp; }
Swaps the rectangles pointed by first two pointers
Changes are permanent since changed through pointers

38. typedef typedef struct rect rect;
typedef associates a type with an identifier
rect is used instead of struct rect after typedef statement
Use typedef after structure declaration